What is the formula to calculate population growth/rate?

P(n)=P(0)e^(kt)

If P(n)=2*P(0) (n years later population will be double of the initial one).

Then ##2=e^(k*t)##

t= years k=population growth rate per year (which is 0.04) Note that there is no limiting factor (or carrying capacity) in this situation.

Compute ##2=e^(k*t)##

##ln 2=t*0.04##

##0.69314718/0.04 = t##

##t=17.33 years##

Again I underline there is no upper limiting factor. Population growth is exponential.